In the field of ophthalmology, as well as in other applications, optical radiation acts inside the material, for example, the tissue, which is transparent to the optical radiation. Non-linear processes are usually used which require a focussing of machining radiation, usually pulsed laser radiation, into the material, i.e., underneath the surface of the material. The production of a cut occurs by displacing the position of the focus in the material. With the knowledge that forms the basis of this description, the shift of the focus does not necessarily require that radiation is also emitted into the focus at this time. In particular when pulsed laser radiation is used, the focus is continuously shifted and laser radiation pulses are only emitted at certain times during the focus shift. Nevertheless the corresponding optics and the focus adjustment device operate continuously, which is why the term “focus shift” herein is also understood to mean the corresponding shift of the point at which optical radiation would be focussed, even if such radiation is momentarily not emitted, e.g., between two laser pulses.
The high focussing of the laser radiation, i.e., a geometrically strongly delimited focus, is of great importance for non-linear effects, as only then can the necessary power densities in the material be achieved. This applies both to non-linear processes in which an individual focus already results in an interaction and to processes in which several laser radiation pulses which are emitted one after the other interact to achieve a material-cutting effect. In this regard, approaches are also known in which laser radiation pulses are emitted at several overlapping focus spots and only the interaction of the several laser radiation pulses leads to material cutting in the overlap area.
Three-dimensional cuts, which extend parallel to the optical axis of the radiation incidence (of the so-called main direction of incidence), are e.g., required as cylindrical-jacket-shaped cuts in the field of ophthalmology, in particular in cataract surgery. Here a circular opening with a particular diameter is produced in the front of the capsular bag. The shape of the cut is then a circular cylinder, which is oriented approximately parallel to the optical axis and thus also parallel to the main direction of incidence of the optical radiation.
EP 1486185 relates to an apparatus for cataract surgery in which the laser radiation is conveyed to the handle with the aid of an optical fiber. A collimator is mechanically adjusted in the handle in order to adjust the position of the focus along the optical axis. The focus can therefore be adjusted along the optical axis only very slowly as rapid movement of the collimator would result in undesired vibrations and thermal loading of the handle. Moreover, the use of an optical fiber at the powers required for eye surgery is extremely problematic.
A method is described in U.S. Pat. No. 7,486,409 and U.S. Pat. No. 6,590,670 for rapidly varying the depth position of a focus on the basis of a vibrating tuning fork. At least one lens of an optical arrangement is fixed to the vibrating arm of a tuning fork which is made to vibrate by an electromagnet. For adjusting the depth, DE 10034251 proposes attaching a corner reflector to the vibrating arm of a tuning fork. The corner reflector is illuminated with a non-collimated light beam and the propagation of the light beam which is reflected back is varied by adjusting the position of the corner reflector. If the light beam is focussed using an objective lens, a rapid variation of the focus position in the depth direction, i.e., along the optical axis, is obtained. These tuning fork arrangements were proposed for optical measurement technology.
For producing three-dimensional cuts which extend parallel to the optical axis of the radiation incidence, laser treatment apparatuses are known which have an optical unit and device for three-dimensional focus adjustment. Such treatment apparatuses shift the focus in the image field and supplement this two-dimensional shift with a shift of the image field plane to obtain a three-dimensional focus position setting. Since the shift of the image field plane is much slower than the two-dimensional shift of the focus position in the image plane, care must be taken with such apparatuses that the image plane shift is needed as little as possible, in order to design the production of cuts to be as rapid as possible. Therefore, for example, spiral-shaped trajectories are known from the state of the art, which combine rapid shift in the image plane with a comparatively slow shift or adjustment of the image plane position. In this way, for example, cylindrical cuts, which lie parallel to the optical axis, can be produced very quickly.
A disadvantage of this approach is that the optics must be designed such that a rapid two-dimensional shift of the focus position is possible in the image field. The image field size must also be designed such that desired cut sizes are covered.